1. Field of the Invention
The present invention relates to an alignment method and apparatus for aligning a cutting blade with a selected street to be cut on a workpiece such as a semiconductor wafer having chip areas defined by a plurality of crossing streets, which are formed on the upper surface of the semiconductor wafer.
2. Related Arts
Referring to FIG. 7, a semiconductor wafer has square chip areas C defined by crossing streets S1 and S2, which run in first and second directions to form a lattice pattern on the upper surface of the wafer. Each square chip area has an integrated circuit pattern formed thereon, and the semiconductor wafer is diced to provide square chips C by cutting the crossing streets S1 and S2. Each square chip is packaged, and the so packaged chip can be used in an electronics device.
To meet the recent demand for reducing electronics in size, thickness and weight, chip size packages (abbreviated as CSPs) have been widely used. In the CSP two or more semiconductor chips are laid on each other and bonded together, and the so layered assembly is laid on a printed board having spherical terminals appearing on its rear side. A resin material is molded to the whole structure to provide a single CSP board 100, as shown in FIG. 8. The CSP board 100 is diced by cutting the crossing streets S1 and S2 to provide packaged devices as large as the semiconductor chip.
In dicing either the semiconductor wafer W and CSP board 100 as shown in FIGS. 7 and 8 respectively, first, the cutting blade is put in alignment with a selected street, and then said selected street is cut. Thereafter, the cutting blade is shifted a street-to-street distance every time the cutting has been completed. It should be noted that alignment of the cutting blade with a selected street is effected only once at the beginning. This step-by-step indexing mode depends on the assumption that all streets run exactly in parallelism.
As a matter of fact, however, streets are liable to be slant more or less. The parallelism of streets is apt to be lowered particularly in a case where a resin material is molded to the CSP board; the CSP board is apt to be deformed when covered with resin. If the cutting is made by moving the cuffing blade a predetermined street-to-street distance in such case that all streets don""t run exactly in parallelism, it may happen that the cutting blade invades the chip area C (see FIG. 7) so that the semiconductor device may be damaged.
To avoid such unfavorable incident, it has been proposed that indexing alignment is repeated prior to the cutting of each and every street (see Japanese Patent H09-52227(A)). This, however, takes much time, lowering the productivity accordingly.
Therefore, there has been an ever-increasing demand for dicing semiconductor wafers with precision even though their streets should not run strictly in parallelism.
To attain this object according to the present invention, in dicing a workpiece having a plurality of chip areas defined by a plurality of streets running and crossing in a first and a second directions and a plurality of alignment spots formed on a surface of the workpiece, the workpiece being diced by a dicing machine including at least a chuck table for fixedly holding the workpiece, the chuck table being rotatable, and being adapted to be put in a controlled angular position; a cutting means having a cutting blade; a feeding means for moving the chuck table and/or the cutting means relative to each other in directions in which required cutting may be effected on the workpiece; and indexing means for moving the chuck table and/or the cutting means a street-to-street distance,
an alignment method for aligning a selected street and the cutting blade with each other, is improved according to the present invention in that it comprises:
a first step of: imaging the surface of the workpiece to detect alignment spots which permit recognition of selected first and second streets running in same direction with at least one street intervening therebetween, determining and storing X- and Y-coordinates of the first and second streets, provided that X-axis is corresponding to a feeding-and-cutting direction whereas Y-axis is corresponding to a feeding-and-indexing direction;
a second step of: determining an angular difference between an inclination of the first street and that of the second street based on the coordinates of the detected alignment spots, obtaining an angle of correction for each street by dividing the angular difference by number of street-to-street spaces existing between the first and second streets, and storing the angle of correction;
a third step of: putting the first and second streets in parallelism with the X-axis to determine the Y-coordinates of intercept of so oriented first and second streets and a distance between the so oriented first and second streets, determining an indexing amount by dividing so determined distance by number of the street-to-street spaces existing between the first and second streets, and storing so determined indexing amount; and
a fourth step of: moving the cutting blade and/or the workpiece in the Y-axial direction based on both of the so determined angle of correction and the indexing amount so that the cutting blade may be put exactly in alignment with a selected street to be cut.
In the first step, at least three alignment spots may be detected for recognizing each of the first and second streets, and the coordinates of the so detected alignment spots are stored;
in the second step, a first linear function representing the first street is determined from the coordinates of the alignment spots for recognizing the first street according to the least squares method; a second linear function representing the second street is determined from the coordinates of the alignment spots for recognizing the second street according to the least squares method; and the angular difference between the inclination of the first street and that of the second street relative to the X-axis is determined from the first and second linear functions; and
in the third step, the first linear function is rotated until the first street has been put in parallelism relative to the X-axis, thereby reducing its angle of inclination to zero, and determining the Y-coordinate of the first street to be the intercept of the so rotated first linear function; and the second linear function is rotated until the second street has been put in parallelism relative to the X-axis, thereby reducing its angle of inclination to zero, and determining the Y-coordinate of the second street to be the intercept of the so rotated second linear function.
The first and second streets may be the opposite outermost streets formed in the workpiece, and the workpiece may be a CSP substrate.
An alignment apparatus to be built in a dicing machine including at least a chuck table for fixedly holding the workpiece, the chuck table being rotatable, and being adapted to be put in a controlled angular position; a cutting means having a cutting blade; a feeding means for moving the chuck table and/or the cutting means relative to each other in directions in which required cuffing may be effected on the workpiece; and an indexing means for moving the chuck table and/or the cutting means a street-to-street distance; the workpiece having a plurality of chip areas defined by a plurality of streets running and crossing in first and second directions and a plurality of alignment spots formed on the surface of the workpiece, is improved according to the present invention in that the alignment apparatus for aligning a selected street and the cutting blade with each other comprises:
an imaging means for taking a picture of the surface of the workpiece, which is fixedly held by the chuck table;
a coordinates detecting-and-storing means for detecting alignment spots which permit recognition of selected first and second streets running in same direction with at least one street intervening therebetween and for determining and storing X- and Y-coordinates of the first and second streets, provided that X-axis is corresponding to a feeding-and-cutting direction whereas Y-axis is corresponding to a feeding-and-indexing direction;
a linear function determining means for determining first and second linear functions ƒ(x) and g(x) representing the first and second streets on the basis of the X- and Y-coordinates of the first and second streets, which are determined in terms of the alignment spots;
a correction angle setting means for determining the angular difference between the first and second linear functions ƒ(x) and g(x) to divide the so determined angular difference by the number of the street-to-street spaces between the first and second streets, thereby determining an angle of correction for each street;
an indexing means for determining the intercepts of the first and second linear functions ƒ(x) and g(x) which have been rotated to be parallel to the X-axis, determining the distance between the first and second linear functions ƒ(x) and g(x), and dividing the so determined distance by the number of the street-to-street spaces between the first and second streets, thereby determining and storing the indexing amount;
a positioning control means for indexing the cuffing blade in the Y-axial direction and for aligning a selected street with the X-axis on the basis of the angle of correction and the indexing amount;
an alignment data storage means connected to the coordinates detecting-and-storing means, the linear function determining means, the correction angle setting means, the indexing means and the positioning means for storing pieces of alignment information pertaining to the workpiece for permitting random access.
In the coordinates detecting-and-storing means, at least three alignment spots may be detected for recognizing the first street and the second street respectively, and the coordinates of the so detected alignment spots may be stored;
In the linear function determining means, a first linear function representing the first street may be determined from the coordinates of the alignment spots for recognizing the first street according to the least squares method; and a second linear function representing the second street may be determined from the coordinates of the alignment spots for recognizing the second street according to the least squares method.
The required alignment can be so controlled as to be in conformity with any amount of deviation by which a selected street is apart from a correct position, relying on the coordinates of the alignment spots of no more than two selected streets, eliminating the necessity of taking pictures of all streets for detecting their alignment spots. The productivity, therefore, cannot be lowered.
The angle of correction and the indexing amount for each street are determined from the linear functions approximating selected streets according to the least squares method, thus assuring that cutting is effected without permitting invasion into any square chip area.
Other objects and advantages of the present invention will be understood from the following description of a cutting machine equipped with an alignment apparatus according to the present invention, which is shown in accompanying drawings.